[RFC PATCH v3 0/6] scsi: release completed passthrough requests before user copyback

[Yingjie Wang <x-professor@outlook.com>]

This rerolls the request-lifetime series after the v2 review:

  https://lore.kernel.org/r/cover.1793003363.git.x-professor@outlook.com

The two BSG copyback fixes and James Houghton's cancellation fix that were
prerequisites for the previous draft are now in the SCSI fixes tree as
a52f2ad45726, ffb7b533a479 and 2b2bb55990cf.  This series is based on
04feb5fd2c23 from that tree.

The affected passthrough paths can copy data to userspace, or unmap a user
buffer that requires copyback, while still retaining a completed request.
If that access faults and the fault needs I/O from the same queue, the task
can wait for a tag held by the request it would release only after the fault
returns.

Changes since v2:

- 1/6 through 4/6 are unchanged apart from rebasing onto the merged
  prerequisites.
- 5/6 adds an io_uring helper that transfers the storage reserved for the
  command's copied SQE to the driver after every SQE field has been consumed.
  The transfer invalidates ioucmd->sqe, rejects multishot users, and makes a
  later -EAGAIN return fail instead of reissuing a command whose SQE storage
  has been overwritten.  It adds neither an allocation nor object growth.
- 6/6 uses 96 bytes of that storage for the bounded SCSI sense snapshot.
  End-I/O saves the result and sense data, queues task work, and returns
  RQ_END_IO_FREE for every completion.  Task work only uses the saved BIO,
  result and sense data; it never accesses the request or scsi_cmnd.  A
  canceled task-work callback or failed sense copy reports a zero sense
  length.

Compared with the v3 plan I posted on October 29, this removes the remaining
sense-bearing case that would have held its tag until task work:

  https://lore.kernel.org/r/20261029.scsi-lifetime-v3-plan.7ad5c4111d3b@outlook.com

It also avoids the extra per-command sense allocation discussed during v2
review.  I am marking this RFC because the new io_uring helper and its
no-reissue contract need review from the io_uring side.

Validation used the exact tip below:

- Each of the six intermediate commits builds independently with W=1 on
  x86-64.  The final tree also builds the affected objects on i386 and on
  x86-64 with CONFIG_IO_URING disabled and SCSI modular.
- A full x86-64 KASAN and lockdep bzImage builds successfully.
- In QEMU with virtio-scsi disk and CD-ROM devices, 20 regression iterations
  passed 960 assertions plus 6 linked-SQE assertions.  The linked test wrote
  a real 18-byte sense snapshot over the reused SQE storage before completing
  the linked command.
- Trace validation passed 100 synchronous and 20 deferred lifecycle windows.
  Every deferred window retained zero completed requests before task work.
  The run recorded 40 CDDA calls and no BUG, WARNING, KASAN, UBSAN or lockdep
  report.
- Four independent exact-image performance runs with successful depth-16
  INQUIRY and a 96-byte response buffer measured 12.825 us/request in normal
  mode and 13.004 us/request with DEFER_TASKRUN.  The existing baseline was
  13.726 and 12.559 us/request, while the sent v2 allocation measured 17.647
  and 16.665 us/request.  These debug-kernel virtual-device numbers show that
  the v2 allocation overhead is absent; their run-to-run spread does not
  support a general performance claim.

The i386 configuration was build-tested but not booted.  CDDA testing covers
the virtual device's expected error path, not audio extraction from real
media.  Transport BSG (block/bsg-lib.c) remains outside this series because
it uses a separate queue and needs an independent dependency analysis.

v1: https://lore.kernel.org/r/cover.1792237341.git.x-professor@outlook.com
Review follow-up: https://lore.kernel.org/r/20261022.scsi-copyback-review.31b8d6a70f4c@outlook.com
CDDA reply: https://lore.kernel.org/r/20261022.scsi-cdda-lifetime.8f31a2c97b42@outlook.com

Yingjie Wang (6):
  scsi: core: release SG_IO request before user copyback
  scsi: core: release legacy ioctl request before user copyback
  scsi: sr: release CDDA request before user copyback
  scsi: bsg: release synchronous request before user copyback
  io_uring/cmd: allow reusing SQE storage for completion data
  scsi: bsg: release io_uring requests from end-I/O

 drivers/scsi/scsi_bsg.c      | 116 +++++++++++++++++++++++------------
 drivers/scsi/scsi_ioctl.c    |  34 +++++++---
 drivers/scsi/sr.c            |   3 +
 include/linux/io_uring/cmd.h |  24 ++++++++
 io_uring/uring_cmd.c         |  22 +++++++
 5 files changed, 151 insertions(+), 48 deletions(-)


base-commit: 04feb5fd2c23894ebc2cdaf4a4cc38b8667b17ac
-- 
2.43.0